A die-casting machine is a molding machine in which an injection plunger is driven to move forward for every shot so as to inject/fill a constant amount of a material of molten metal such as an Al alloy or an Mg alloy into a mold cavity to thereby mold a product with a required shape. In the same manner as an injection molding machine which injects/fills a plastic material into a mold cavity so as to mold a product with a required shape, the die-casting machine injects/fills a molding material into a mold cavity in a low-speed injection step, a high-speed injection step and an intensification step (corresponding to a holding pressure step in the injection molding machine). However, the die-casting machine is characterized in that the injection speed in the high-speed injection step is about one digit higher than that in the injection molding machine. Therefore, in the background art, a hydraulic die-cast machine in which an injection plunger is driven by hydraulic pressure has been the mainstream.
However, the hydraulic die-casting machine is apt to contaminate a molding factory with oil so that the working environment may deteriorate. Therefore, in recent years, electrically driven die-casting machines without such a drawback have been proposed (for example, see Patent Literatures 1 and 2).
An electrically driven injection device disclosed in Patent Literature 1 is provided with two injection electrically driven servo motors 101a and 101b, and torques of the two injection electrically driven servo motors 101a and 101b are converted into linear forces of nuts 102a and 102b by ball screw mechanisms 104a and 104b consisting of the nuts 102a and 102b and screw shafts 103a and 103b threaded thereon so as to move an injection plunger 106 forward/backward by means of a moving member 105 to which the nuts 102a and 102b are attached integrally, as shown in FIG. 5. In addition, a surge pressure preventing device (hydraulic cylinder 107) is provided between the moving member 105 and the injection plunger 106 so as to prevent excessive surge pressure from acting on a molten metal material in a mold cavity when a high-speed injection step is completed. In this electrically driven injection device, the injection plunger 106 is driven by a resultant force of the two injection electrically driven servo motors 101a and 101b. Thus, a high injection speed can be obtained. In addition, due to the surge pressure preventing device 107 provided between the moving member 105 and the injection plunger 106, a good product without poor appearance such as burrs can be molded, and a mold or the like can be prevented from being broken by surge pressure.
On the other hand, in an electrically driven injection device disclosed in Patent Literature 2, as shown in FIG. 6, the torque of an injection electrically driven servo motor 201 is transmitted to a screw shaft 204a of a ball screw mechanism 204 through a speed reducer 202 and a friction clutch 203 so that an injection plunger 205 can be moved forward/backward by a nut 204b of the ball screw mechanism 204 threaded on the screw shaft 204a. In this electrically driven injection device, the friction clutch 203 put between an output shaft of the speed reducer 202 and the screw shaft 204a of the ball screw mechanism 204 prevents excessive surge pressure from acting on a molten metal material in a mold cavity when a high-speed injection step is completed. Thus, a good product without poor appearance such as burrs can be molded, and a mold or the like can be prevented from being broken by surge pressure.